The invention relates to 2-dimensional position sensors. More particularly the invention relates to 2-dimensional position sensors of the type based on capacitive proximity sensing techniques. Such sensors may be referred to as 2-dimensional capacitive transducing (2DCT) sensors. 2DCT sensors are based on detecting a disturbance in a capacitive coupling of sensor electrodes, either to ground or to another electrode, caused by the proximity of a pointing object. A measured location for the disturbance corresponds to a measured position for the pointing object.
2DCT sensors are typically actuated by a human finger, or a stylus. Example devices include touch screen and touch sensitive keyboards/keypads, e.g. as used for controlling consumer electronic devices/domestic appliances, and possibly in conjunction with an underlying display, such as a liquid crystal display (LCD), or cathode ray tube (CRT). Other devices which may incorporate 2DCT sensors include pen-input tablets and encoders used in machinery for feedback control purposes, for example.
Devices employing 2DCT sensors have become increasingly popular and common, not only in conjunction with personal computers, but also in all manner of other appliances such as personal digital assistants (PDAs), point of sale (POS) terminals, electronic information and ticketing kiosks, kitchen appliances and the like. 2DCT sensors are frequently preferred to mechanical switches for a number of reasons. For example, 2DCT sensors require no moving parts and so are less prone to wear than their mechanical counterparts. 2DCT sensors can also be made in relatively small sizes so that correspondingly small, and tightly packed keypad arrays can be provided. Furthermore, 2DCT sensors can be provided beneath an environmentally sealed outer surface/cover panel. This makes their use in wet environments, or where there is a danger of dirt or fluids entering a device being controlled attractive. Furthermore still, manufacturers often prefer to employ interfaces based on 2DCT sensors in their products because such interfaces are often considered by consumers to be more aesthetically pleasing than conventional mechanical input mechanisms (e.g. push-buttons).
2DCT sensors may be considered to broadly fall into two categories. Namely those based on passive capacitive sensing techniques, and those based on active capacitive sensing techniques.
Passive capacitive sensing devices rely on measuring the capacitance of a sensing electrode to a system reference potential (earth). The principles underlying this technique are described in U.S. Pat. No. 5,730,165, and U.S. Pat. No. 6,466,036, for example. The contents of U.S. Pat. No. 5,730,165 and U.S. Pat. No. 6,466,036 are incorporated herein in their entirety by reference as describing background material to the invention. In broad summary, passive capacitive sensors employ sensing electrodes coupled to capacitance measurement circuits. Each capacitance measurement circuit measures the capacitance (capacitive coupling) of its associated sensing electrode to a system ground. When there is no pointing object near to the sensing electrode, the measured capacitance has a background/quiescent value. This value depends on the geometry and layout of the sensing electrode and the connection leads to it, and so on, as well as the nature and location of neighbouring objects, e.g. the sensing electrodes' proximity to nearby ground planes. When a pointing object, e.g. a user's finger, approaches the sensing electrode, the pointing object appears a virtual ground. This serves to increase the measured capacitance of the sensing electrode to ground. Thus an increase in measured capacitance is taken to indicate the presence of a pointing object.
U.S. Pat. No. 5,730,165 and U.S. Pat. No. 6,466,036 are primarily directed to discrete (single button) measurements, and not to 2D position sensor applications. However the principles described in U.S. Pat. No. 5,730,165 and U.S. Pat. No. 6,466,036 are readily applicable to 2DCT sensors, e.g. by providing electrodes to define either a 2D array of discrete sensing areas, or rows and columns of electrodes in a matrix configuration.
Passive sensing techniques have been found to be very useful and reliable in a number of applications. However, there are some drawbacks. For example, passive 2DCT sensors are strongly sensitive to external ground loading. That is to say, the sensitivity of such sensors can be significantly reduced by the presence of nearby low impedance connections to ground. This can limit their applicability. For example, some types of display screen technology provide for a low-impedance coupling to ground across the visible screen. This means a passive 2DCT overlaying the display screen will often under-perform because the relatively strong coupling to ground through the screen itself reduces the sensitivity of the 2DCT to any additional coupling to ground caused by an approaching pointing object. A similar effect means 2DCT sensors can be relatively sensitive to changes in their environment, e.g., a 2DCT sensor might behave differently according to its location because of differences in capacitive coupling (ground loading) to external objects. 2DCT sensors are also relatively sensitive to environmental conditions, such as temperature, humidity, accumulated dirt and spilt fluids, etc. All of these effect the sensor's reliability and sensitivity. Furthermore, the measurement circuitry associated with passive 2DCT sensing is generally of high input impedance. This makes passive sensors prone to electrical noise pick up, e.g. radio frequency (RF) noise. This can reduce reliability/sensitivity of the sensor and also places constraints on sensor design (e.g. there is limited freedom to use relatively long connection leads/traces between the sensing electrodes and associated circuitry.
Active 2DCT sensors, on the other hand, are less prone to the above-mentioned effects associated with passive 2DCT sensors. Active 2DCT sensors are based on measuring the capacitive coupling between two electrodes (rather than between a single sensing electrode and a system ground). The principles underlying active capacitive sensing techniques are described in U.S. Pat. No. 6,452,514. The contents of U.S. Pat. No. 6,452,514 are incorporated herein by reference in their entirety as describing background material to the invention. In an active-type sensor, one electrode, the so called drive electrode, is supplied with an oscillating drive signal. The degree of capacitive coupling of the drive signal to the sense electrodes is determined by measuring the amount of charge transferred to the sense electrode by the oscillating drive signal. The amount of charge transferred, i.e. the strength of the signal seen at the sense electrode, is a measure of the capacitive coupling between the electrodes. When there is no pointing object near to the electrodes, the measured signal on the sense electrode has a background/quiescent value. However, when a pointing object, e.g. a user's finger, approaches the electrodes (or more particularly approaches near to the region separating the electrodes), the pointing object acts as a virtual ground and sinks some of the drive signal (charge) from the drive electrode. This acts to reduce the strength of the component of the drive signal coupled to the sense electrode. Thus a decrease in measured signal on the sense electrode is taken to indicate the presence of a pointing object.
A 2DCT active sensor described in U.S. Pat. No. 6,452,514 comprises drive electrodes extending in rows on one side of a substrate and sense electrodes extending in columns on the other side of the substrate so as to define an array of N by M touch keys. Each key corresponds to an intersection between a drive electrode and a sense electrode. Thus the array of keys described in U.S. Pat. No. 6,452,514 may be termed a matrixed array with a single drive electrode (i.e. a single conductive element connected to a single drive channel) associated with all keys in a given column and a single sense electrode (i.e. a single conductive element connected to a single sense channel) associated with keys in a given row. This reduces the number of drive and sense channels required since a single drive channel simultaneously drives all of the keys in a given column and a single sense channel senses all of the keys in a given row. The capacitive coupling between the electrodes at the positions of the different keys can be determined by driving the appropriate column and sensing the appropriate row. For example, to determine the capacitive coupling between the electrodes associated with a key at the intersection of column 2 and row 3, the drive signal is applied to the drive electrode of column 2 while the sense channel associated with the sense electrode of row 3 is active. The output from the active sense channel reflects the capacitive coupling between the electrodes associated with the key under investigation. Different keys can be scanned by sequencing through different combinations of drive and sense channels. In one mode the drive electrodes may be driven sequentially while the sense electrodes are all continuously monitored. A signal change on one (or more) of the sense electrodes indicates the presence of a pointing object. The sense electrode on which the change is seen defines position in one dimension, the drive electrode being driven when the change was seen defines position in the other dimension.
U.S. Pat. No. 5,648,642 also discloses a 2DCT sensor based on active capacitive sensing. This sensor operates according to broadly the same basic principles as described in U.S. Pat. No. 6,452,514. The sensor of U.S. Pat. No. 5,648,642 is schematically shown in FIGS. 1A, 1B and 1C. These figures respectively show top, bottom and composite views of the sensor. The sensor 10 comprises a substrate 12 including a set of first conductive traces 14 disposed on a top surface 16 thereof and run in a first direction to comprise column electrodes of the sensor 10. A set of second conductive traces 18 are disposed on a bottom surface 20 thereof and run in an orthogonal second direction to form row electrodes of the sensor array 10. The sets of first and second conductive traces 14 and 18 are alternately in contact with periodic sense pads 22 comprising enlarged areas, shown as diamonds. A 0.254 cm center-to-center diamond-shaped pattern of sense pads disposed along a matrix of 15 rows and 15 columns of conductors is employed. Every other sense pad 22 in each direction in the pad pattern is connected to sets of first and second conductive traces 14 and 18 on the top and bottom surfaces 16 and 20, respectively of substrate 12.
The 2DCT sensor shown in FIGS. 1A to 1C may thus be operated in an active mode in which the columns 14 of connected sense pads 22 shown in FIG. 1A comprise respective drive electrodes, and the rows 18 of connected sense pads 22 shown in FIG. 1B comprise respective sense electrodes. These may be scanned in a sequential manner as described both in U.S. Pat. No. 5,648,642 and also in U.S. Pat. No. 6,452,514.
Thus 2DCT sensors based on active capacitive proximity sensing may provide sensors which in some circumstances can be more reliable than 2DCT passive sensors. Furthermore, a matrix array of drive and sense electrodes, such as shown in FIGS. 1A to 1C, may be employed instead of an array of discrete self-contained drive and sense electrode pairs. This has the advantage of reducing the number of connections required to be made between the electrodes comprising the 2DCT sensor and the associate drive/sense circuitry. This not only makes for simpler wiring logistics, it also reduces cost because fewer drive/sense channels are required, e.g. a 2DCT sensor comprising an array of N×M sensing areas requires N drive channels and M sense channels in a matrix configuration, but (N×M) of each in a discrete sensing area configuration.
When implementing drive and sense channel circuitry of the kind described in U.S. Pat. No. 6,452,514 in an integrated circuit (IC) chip package, each drive channel requires one pin-out while each sense channel requires two pin-outs. Thus, for a 2DCT sensor comprising an array of n×m sensing areas, a matrixed array requires N+2M pin outs (or M+2N pin outs depending on which of the columns and rows are drive or sense electrodes—i.e. which are connected to drive or sense channels). However, a discrete (non-matrixed array) requires 3NM pin outs. Circuit connections, and in particular pin outs in IC chip implementations, are expensive, both in monetary terms, and in terms of the physical space and complexity required to implement them.
Thus there is a desire to provide a 2DCT sensor based on active capacitive sensing techniques which requires still fewer connections, i.e., requiring still fewer external connections compared to known 2DCT sensors based on matrixed arrays.